Combined TPR, XRD, and FTIR studies on the reduction behavior of Co3O4

The test (Co3O4) and two control cobalt oxides (CoO and Co2O3) were the study materials of the present investigation. The test oxide was a high purity (≥99%) commercial material, but the control oxides were laboratory-prepared. Their chemical composition and structure were determined by X-ray diffractometry and Fourier-transform infrared spectroscopy, thermal stability, and topo-chemical interactivity by thermogravimetry (TG) in pure N2 or O2 atmosphere, hydrogen-reduction behavior, events, and products by non-isothermal and isothermal temperature-programmed reduction measurements. Results obtained show the Co3O4 to assume a normal-spinel-type crystalline structure, which remains thermally stable till the onset of its complete decomposition into CoO (>1050–1200 K) in the N2 atmosphere. In the O2 atmosphere, however, it suffers topo-chemical deoxygenation down to Co3O4-0.024 (≤680 K) succeeded by oxygenation up to Co3O4+0.05 (>680 K) till the immediate vicinity of its bulk decomposition into CoO at the higher temperature regime of 1204–1223 K. Co3O4 is the immediate product of bulk CoO oxidation (at 570–1170 K) or bulk Co2O3 decomposition (480–590 K). The H2-reduction of Co3O4 is shown to occur via five events involving removal of weakly bound surface oxygen species (at 493–680 K), reduction into CoO (703–798 K), reduction of the thus formed CoO into hexagonal and/or cubic crystallites of Co0 via formation (and subsequent reduction) of charged Co5O clusters (853–1100 K), whose peak temperatures were found to depend on the reduction conditions applied. Therefore, impacts of reduction variables (oxide mass, heating rate, and reducing gas composition and flow rate) were systematically examined and found to alter significantly the temperature regimes and rate of the reduction events. Accordingly, the activation energy was calculated for each reduction event and found to impart Co0-inflected autocatalytic effects. Eventually, it is believed that joint Co2+-O2--Co3+ linkages established solely in Co3O4 might be behind the distinctive high thermal stability, O2-inflected topo-chemical activity, and reduction behavior of Co3O4 versus CoO and Co2O3. These linkages seem to tolerate reversible redox cycles (Co2+↔ O2− ↔ Co3+), as may be implied from TG-observed cyclic inter-conversion of CoO ↔ Co3O4 ↔ CoO in the O2 atmosphere.